1. Field of the Invention
This invention pertains to measuring instruments, and more particularly relates to digital pressure gauges and solar powered devices.
2. Description of the Prior Art
The most familiar use of solar powered devices is the solar calculator. Calculators have a “clear” button that clears and zeros the display, should the display not start at zero. The user interacts with a keyboard to input values and start operations. An LCD (liquid crystal display) displays the resulting number. The first device that operated independent of a user intervention is disclosed by Jamieson, in U.S. Pat. No. 5,196,281, the disclosure of which is incorporated herein by reference. Jamieson describes a digital thermometer with a thermo-resistive sensor powered entirely by solar power capable of turning on and turning off with the available light. While solar powered calculators, watches, and thermometers are in common use today, the application to the measurement of pressure has remained elusive. Pressure sensors that exhibit a capacitance change with pressure offer the best advantage for low power operation that is necessary for solar powered devices where only a few microwatts are available for power.
Lee, in U.S. Pat. No. 3,859,575, the disclosure of which is incorporated herein by reference, describes a capacitive sensor for a pressure transducer. The construction attempts to minimize the changes in capacitance at zero pressure due to differential thermal expansion. A few years later, Lee described a “Center-Mounted Capacitive Pressure Sensor” in U.S. Pat. No. 5,542,300, the disclosure of which is incorporated herein by reference, wherein is described a method to adjust for the ambient temperature effect due to differential thermal expansion. Since with this design the travel is small, it is most important to maximize the dimensional stability. To provide higher pressure ranges, the deformable section, the diaphragm, is made stiffer by increasing the thickness. Capacitive pressure sensors are widely used as the sensor in digital oscilliometric blood pressure monitors as they offer low power and low cost. Capacitive sensors for measuring force are in common use with bathroom scales. Two such scales are known to this inventor that also employ solar cells for power: Model 1610 made by Tanita Corporation of Tokyo, Japan and Model 8100 made by Taylor Precision Products LP of Oak Brook, Ill. The blood pressure monitors and the scales both use an auto-zero routine before the measurement is made to offset the sensor capacitance change with environmental conditions such as temperature and humidity. In the case of the bathroom scale, the auto-zero also compensates for the error due to light level conditions, assuming that the light is consistent before and during the measurement cycle. Lew, in U.S. Pat. No. 5,317,918 and Delatorre in U.S. Pat. No. 5,230,250, the disclosures of which are incorporated herein by reference, describe sensors which use the force from diaphragms, bellows or Bourdon tubes to pull or torque separate capacitive elements to change the gap and effect the capacitance. In all of these cases, two flat plate electrodes face each other separated by a small gap forming a basic capacitor. As the two plates separate, the capacitance diminishes according to the inverse relation of the distance between the plates. This basic geometric arrangement defines the limitations of the approach thus far. The distance relation between the electrode pairs in a capacitive sensor is the focus of the present invention that takes advantage of the large free motion of Bourdon tubes.
Pressure gauges are subject to a variety of environmental conditions, such as ambient temperature, humidity and varying light levels. As with the thermometer, previously referred to, the pressure is most likely not at zero when the gauge is powered on. Advantages from having auto-zeroing cannot be realized. Since many pressure gauges are mounted to pipelines, tanks or tubing in close proximity to pumps and equipment, they are subjected to vibration and pulsation. A common practice is to liquid-fill a gauge with glycerin or silicone oil to dampen the movement and steady the pointer so that a reading can be made even when the gauge is shaking. Pressure gauges that have a sealed case to keep out the elements, or that are liquid filled, will often exhibit an error due to temperature changes that cause the case pressure to change; case compensators such as flexible diaphragms or bladders are used to reduce these effects.
With the globalization of all markets, it has become common for pressure instruments to have multiple scales (for example: PSI, Bar, kg/cm2, KPa). Standardization has not been effective in pressure measurement throughout the world. Mechanical gauges often have scales of concentric arcs, sometimes with 2, 3, or 5 scales, and as such, the readability of the inner scales becomes poor. Electronic pressure instruments have an advantage of being capable of selecting a variety of units of measure. The control integrated circuit “IC” is programmed to calculate the conversion from one unit to another.
Refrigeration gauges, for example, are used for servicing and filling systems. They all have multiple scales, each corresponding to the vapor temperature/pressure relation of a particular refrigerant. The concern over CFCs (chlorofluorocarbons) and their effect on the environment has propelled the development of many new refrigerants. The refrigeration gauge indicates the vapor temperature that corresponds to the vapor pressure of the particular refrigerant, either in degrees F. or degrees C., in a logarithmic, nonlinear, expanding scale resolution. A typical refrigeration gauge would have a scale resolution several times smaller at the top of the scale than at the bottom of the scale. At zero degrees F., the scale increments may be five degrees, and at 100 degrees F., only one degree as shown by the minor divisions in the scale arc. With so many new refrigerants, it is difficult to keep refrigeration gauge scales current. An electronic gauge has the advantage to be able to select multiple scales. Conventional pressure gauges can also select the scale or unit of measure and “dial in” to display the reading in lb/in2, Kg/cm2, Bar's, mmHg, inHg or KPa.
The state of the art thus far for the electronic pressure gauge is limited to external power sources or battery power. Battery powered gauges often have an auto-off feature to save battery life. This necessitates user interaction to turn the gauge on; traditionally, this is not familiar to the user who is accustomed to just observing the pointer position on a mechanical gauge. A light powered pressure gauge that can operate in low levels of light can have the readability and versatility of a digital gauge and has the benefit of always appearing on without the need for user intervention.
The requirements of a solar powered pressure gauge are rigorous, especially considering the requirements of low power, wide range of light conditions, high accuracy, environmental compatibility, ambient temperature effects, over pressure capability and the vast variety of sizes and pressure ranges that need to be offered.